Field of the Invention
The present invention relates to an amplifier circuit, and in particular relates to a fully differential amplifier of class A/AB type and a method thereof.
Description of the Related Art
An amplifier modulates current from a power supply responsive to an input signal. Amplifiers are divided into classes based on the circuits by which a proportion of the input signal is amplified, e.g. Class A, Class B and Class AB amplifiers.
For a Class A amplifier, the active element remains active over the entire conduction cycle of the input signal, consuming power almost all the time. As for a Class B amplifier, the active element remains active over the half of the conduction cycle, and the distortion in the output signal thereof is noticeable.
A Class AB amplifier is an amplifier which contains push and pull devices, each is biased to conduct more than half of the conduction cycle, reducing cross-over clipping of a Class B amplifier at the expense of an increased power consumption associated with a Class A amplifier. The increased power consumption arises from what is known as a “quiescent current” of the Class AB amplifier, which flows from positive to negative power supplies through both the push and pull devices in the absence of the input signal. The Class AB type amplifier requires only small amounts of quiescent currents to reduce cross-over distortion considerably, therefore are widely adopted in audio, signal processing, and radio frequency applications.
Accordingly, the choices of the quiescent currents impact considerably on the level of distortion and the power efficiency. When the push and pull devices of the Class AB amplifier are operated by voltage biasing, the quiescent currents through the push and pull devices are determined by bias voltages, device process, temperature, and power-supply voltage variation. The mismatched quiescent currents through the push and pull devices may lead to circuit performance degradation such as increased common mode DC offsets and increased differential mode DC offsets.